This invention is directed generally to a control system for operating a hydraulically driven tool. More particularly, the invention is directed to a novel control valve arrangement for such a tool which may be used in both constant pressure and constant volume hydraulic fluid systems without requiring the disassembly or replacement of any parts in the valve or in the hydraulically driven tool.
Generally speaking, two basic types of hydraulic systems are used as the power source for hydraulically operated tools, namely, the constant volume system and the constant pressure system. In the constant volume system, in the off or neutral position, the hydraulic oil or other fluid must be free to flow back to the power source. Accordingly, the on-off hydraulic control valves for use with such a system have heretofore employed a so-called open center spool so that when the valve is in its off or neutral position the fluid is free to flow through the valve and back to the source. In the constant pressure system, the hydraulic pump operates only intermittently to achieve and maintain a desired pressure. Accordingly, with a constant pressure system, the on-off control valve has heretofore employed a so-called closed center spool so that in the off or neutral position the flow of fluid is blocked to maintain the desired system pressure.
Hydraulically driven tools are used in many applications in the field, for example, by utility companies, municipalities, park districts, and the like. In particular, such tools are utilized in arbor or foliage management and hence, such tools are often utilized for example, by tree management companies, orchards, and the like as well. While the present invention is directed to a tree pruner type of tool, it will be appreciated that the invention may find application in other sorts of hydraulically operated tools.
Many of the foregoing users of such tools frequently employ both constant pressure type and constant volume type hydraulic power sources. That is, various equipment such as trucks and the like utilized in the field may be equipped with one or the other of these types of hydraulic power sources or systems. Accordingly, many of the users of such tools have heretofore found it necessary or desirable to maintain duplicate sets of tools in order to operate with either type of system. For example, if in the field only hydraulic tools for operation with one of these two types of systems were available, workers would be required to return to the shop if by chance the hydraulic power system provided or employed in the field was not of the same type as the tools which had been provided.
While providing duplicate sets of tools overcomes this difficulty, this represents quite an increase in capital costs, storage costs and the like. Employing and maintaining duplicate sets of tools also takes up much needed space on trucks or other equipment-carrying vehicles or the like.
One possible solution would be to design tools with interchangeable valve spools, one with an open center and one with a closed center. The operator could then select and install the proper spool to match the hydraulic power source. However, this would require that duplicate spools be available for use with each tool, again requiring additional inventory and storage costs and space requirements. Moreover, it would require the Operator have sufficient skills to properly disassemble and reassemble the valves with each of the tools to be used. This again requires some additional time at the job site for disassembly and reassembly of the valves of various tools as may be necessary. The frequent removal and replacement of valve tools would also unnecessarily disturb the hydraulic system and seals and the like and provide increased opportunities for dirt, debris and the like to be introduced into the hydraulic system. As a related matter, the spools which were removed from the tools and not in use would be left unprotected and subject to damage, loss, contamination, and the like.
The prior art has proposed two solutions to the foregoing problems in the form of valve assemblies which were designed to work in either a normally closed or normally open position. U.S. Pat. No. 3,882,883 discloses a valve assembly having a spool which may be rotated 180.degree. to shift from a normally open operating mode to a normally closed operating mode. However, this valve design requires that a linkage rod be removed before the spool may be rotated. Thus, there is still the possibility of the linkage rod being improperly removed and reassembled, lost, damaged, contaminated or the like.
In U.S. Pat. No. 4,548,229 a valve assembly for accommodating both open center and closed center modes of operation is disclosed for use with an impact wrench. However, this valve assembly is suitable only for use with rotating tools, because the valve assembly itself is designed to shunt hydraulic fluid back to the source when the tool is in the off or neutral state, in the open center mode of operation. This tool is provided with a specially designed valve cylinder or sleeve which surrounds the valve spool. The sleeve is configured for open center operation when in a first orientation and for closed center operation when it is rotated to a second orientation approximately 180.degree. of rotation from its first orientation. This valve is designed to permit constant flow of hydraulic fluid through the tool when the valve is in its on position in both open center and closed center modes of operation. The valve is designed to cut off the hydraulic fluid flow at the valve itself in the closed center mode of operation when the valve is in its closed or neutral position. Conversely, this valve is designed to direct flow through the valve itself and back to the source when in the off or neutral position in its open center mode of operation.
That is, in U.S. Pat. No. 4,548,229, in both open center and closed center modes, when the valve is in its off or neutral position, the valve does not permit flow of fluid past the valve and there is no fluid flow to the tool. However, such a valve arrangement will not work with a reciprocating type of hydraulic power tool wherein it is necessary to alternately direct flow to opposite sides of a reciprocating piston. The tree pruner or lopper device disclosed in connection with the present invention is one such type of tool which utilizes a reciprocating piston, rather than a rotating rotor as utilized in tools such as the impact wrench of the above-mentioned U.S. Pat. No. 4,548,229.